As will be shown and described below, the storage and separation in accordance with the present invention may be applied in any of a variety of multipoles that function as collision cells or ion traps. One example of a multipole in which the embodiments of the present invention may be applied is an RF-only multipole. RF-only multipole structures are widely used in mass spectrometers as ion guides and/or collision cells. Generally described, RF-only multipoles consist of four or more elongated rods that bound an interior region through which ions are transmitted. The ions enter and exit the multipole rod set axially. A radio-frequency (RF) voltage is applied to opposed rod pairs to generate an RF field which confines the ions radially and prevents ion loss arising from collision with the rods. RF-only multipoles are operationally distinguishable from standard quadrupole mass filters, which utilize a DC electric field component in the radial direction to enable separation of ions according to mass-to-charge (m/z) ratio. As the name denotes, RF-only multipoles omit the DC field component in the radial direction and thus allow passage of ions having differing m/z ratios.
In many mass spectrometers, the ion source (such as an electrospray ionization (ESI) source, an atmospheric pressure chemical ionization (APCI) source, as well as certain types of matrix-assisted laser desorption ionization (MALDI) sources) operates at a significantly higher pressure relative to the pressure in the mass analyzer region. Due to collisional damping effects (which reduce the kinetic energy and improve transmission efficiency of ions within the multipole) it may be desirable or necessary to provide an axial DC field in an RF-only multipole located in a high-pressure or intermediate-pressure region within a mass spectrometer to assist in propelling the ions along the longitudinal axis of the multipole. Generation of the axial DC field is commonly achieved by using (i) segmented RF-only multipoles with variable DC offset voltages between segments; (ii) tilted or shaped appropriately auxiliary metal rods positioned in gaps between RF rods; or, (iii) a set of supplemental auxiliary rods (metal segments or isolator covered with resistive material), located between the main RF rods and being arranged substantially parallel thereto. In the last case, an axial DC potential gradient is created by applying a first voltage to corresponding first ends of the auxiliary rods and a second voltage to corresponding second (opposite) rod ends. The use of auxiliary rods and related techniques for generating an axial DC field in RF-only multipoles is disclosed in, for example, U.S. Pat. No. 6,111,250 by Thomson et al., entitled “Quadrupole with Axial DC Field.”
The implementation of auxiliary rods in RF-only multipoles may complicate the operation of transfer optics in mass spectrometers. A notable operationally significant challenge is that the DC potential in the radial plane orthogonal to the major longitudinal axis of the multipole may vary significantly with angular and radial position, being dependent upon the geometry of both rod sets and the differences in DC voltages applied. Poor homogeneity of DC potential may adversely affect ion transmission efficiency because of high order (such as octopole) DC fields, especially when large excursion of ion trajectories from the major longitudinal axis occurs.
Because of these problems, there is a need to be able to better control ion movement and storage throughout a multipole. This is especially true when it is desired to handle large numbers of ions and/or ions having a large range of m/z. Relatedly, there is a need to be able to efficiently and selectively eject ions axially from the multipole. These needs are pertinent to a variety of multipoles including those that have radial DC components as well as in RF-only multipoles. Many patents have addressed related details, but have done so for other purposes, as may be noted in the following paragraphs.
U.S. Pat. Nos. 6,177,668; 7,041,967; 6,504,148; 7,019,290; and 6,703,670 to Hager et al. have methods and apparatuses that depend on use of an RF or AC induced fringing field for axial ejection. These patents teach selectively moving or oscillating ions within the multipoles of traps, filters or an ion guide by an auxiliary RF field. These patents disclose ejecting ions axially by utilizing fringing fields and auxiliary RF fields.
U.S. Pat. No. 7,045,797 to Sudakov et al. (and prior publication of US Patent Publication 2004/0108456) has axial ejection that is effectuated by moving the ions into a fringing field similar to the disclosure of Hager '668 or by a DC modulation to bring ions into resonance with an AC excitation field.
The U.S. Pat. Nos. 5,847,386 and 6,111,250 to Thomson et al. and the U.S. Pat. No. 6,163,032 to Rockwood have opposite axial DC fields caused by pairs of rods forming transverse planes that generally appear to intersect on the central axis of a multipole. The opposite fields are effectuated by rods that are sloped or tapered in opposite directions along the central axis. However, the disclosures of Thomson et al. teach that the axial fields are for transport of ions axially through fringing fields. Rockwood has an improvement to the Thomson configuration in which Rockwood teaches removing quadrupolar and high band pass effects that are typically induced by applying axial DC fields to a multipole.
U.S. Pat. No. 6,713,757 to Tanner et al. has tapered and/or sloped rods in a multipole for axially moving ions similar to the Thomson and Rockwood patents.
US Patent Application, Publication No. 2005/0253064 A1 to Loboda et al. and U.S. Pat. No. 7,084,398 B2 to Loboda et al. utilize AC fields for containing ions radially and static axial fields for urging ions axially. Segmentation or coating of the rods as well as auxiliary electrodes for applying a DC gradient voltage along a length of the rods is disclosed. Also, both AC and DC voltages are applied to each of the segments. That is, Loboda appears to apply a DC voltage gradient in a set of rods and at the same time maintain a main RF that acts in a conventional manner to contain the ions. The references to Loboda also describe controlling distribution of ions in a multipole.
U.S. Pat. No. 7,049,580 to Londry et al. is directed to fragmentation and has shaped rods for controlling axial movement of ions. However, the voltages applied to diametrically opposed pairs of the rods are identical to each other. Londry also teaches damping with higher order multipole configurations to inhibit ejection of ions from a trap.
U.S. Pat. Nos. 5,679,950 and 5,783,824 to Baba have shaped or segmented rods. Baba 5,783,824 has auxiliary rods installed in gaps between adjacent main RF rods. Axial DC voltage gradients are applied to the shaped auxiliary rods. This and/or an additional RF voltage creates an overall harmonic axial field for oscillating and ejecting ions axially.
U.S. Pat. No. 5,576,540 to Jolliffe discloses moving ions off center, but regularly ejects ions radially. Jolliffe's disclosure does state that the direction of ejection does not matter. Jolliffe also describes adjusting the DC field in a manner that will not interfere with the fringing fields.
U.S. Pat. No. 7,067,802 to Kovtoun has a resistive path forming an axial DC gradient on RF only rods.
U.S. Pat. No. 6,791,078 B2 to Giles et al. has a mobility separator mass spectrometer. This device uses a transient DC voltage to move ions axially
U.S. Pat. No. 6,833,544 to Campbell et al. has axial ejection as is apparent from the Figures and description. Campbell et al. has an axial DC field that is created by DC voltage gradients applied to segmented rods. The disclosure of Campbell et al. teaches ejection by increasing the amplitude of the auxiliary RF.